Benchmark: capture errors/artifacts in build.yml, revert throughput chart

- build.yml: upload the build-vs-release benchmark outputs (JMeter HTML reports,
  *.jtl, docker logs, *.jmeter.out) as artifacts with `if: always()` and 90-day
  retention in both build-docker and build-docker-alpine, so benchmark failures
  are diagnosable from the run.
- compare-opendj.sh: stop discarding JMeter output (write it to <slug>.jmeter.out)
  and print a per-image error breakdown (count | op | code | message) parsed from
  the .jtl to the step log.
- summary.sh: revert the Total throughput chart back to vertical bars (keep the
  tests/s label).